Bias tires having relatively narrow cross sectional widths were predominantly used in vehicles up to around 1960 because vehicles in those days were relatively lightweight, had relatively low cruising speed required thereof and thus did not put so much stress on the tires. However, radial tires having wide and flat structures are predominant these days because good driving stability in high speed running, as well as good wear resistance, is required of tires as highway networks are developed and vehicle speed increases (e.g. PTL 1).
However, increasing widths of tires decreases free space in a vehicle and deteriorates comfortablility therein. Further, wider widths of tires increase air resistance and cause another problem of poor fuel efficiency. There has been increasingly a demand for a better fuel efficiency in recent years as people are more concerned about environmental issues.
Electric vehicles which are being developed for use in future, in particular, must have sufficient space for accommodating driving units such as a motor for controlling torque of rotating tires around drive shafts. Ensuring sufficient space in vicinities of tires is becoming increasing important in this connection.
Further, such a wide and flat tire as described above exhibits relatively poor drainage performance because the tire has a relatively wide (ground contact) leading surface and thus water is not smoothly drained on respective sides of the tire in running in a wet condition, as schematically shown by arrows representing water flow lines in FIG. 1A. Yet further, the wide and flat tire is susceptible to what is called hydroplaning phenomenon because the tire, having a relatively short ground contact length L, allows a water film entering from the leading surface to float a ground contact surface up, so that an actual ground contact area and thus gripping force decrease, as shown in FIG. 1A. In short, the wide and flat tire has a problem of deteriorated wet performance, as well.
In view of the deteriorated wet performance, a conventional radial tire having a wide and flat structure in particular must have a main groove extending in the tread circumferential direction and/or a lug groove extending in the tread width direction each formed in a ground contact surface of a tread to have a relatively large cross-sectional area, in order to ensure good drainage performance.
However, in a case where a main groove and/or a lug groove having wide groove widths are provided in a tread, there arises a problem in that a negative ratio of the tread increases to decrease a ground contact area and gripping force, thereby deteriorating driving stability and braking performance on a dry road surface and also reducing wear resistance and worsening noise. Further, in a case where grooves having deep groove depths are provided in a tread, tread rubber must be thick accordingly, which increases tire weight and thus deteriorates running performance of the tire.
It is conventionally known that use of tread rubber having relatively low hysteresis loss for a wide and flat radial tire is effective in terms of reducing rolling resistance and thus improving fuel efficiency of the tire. However, use of rubber having relatively low hysteresis loss for a tire then causes a problem in that gripping performance of the tire on a wet road surface deteriorates.